Health Briefs

September 6, 2002

Physical Activity Among Teenaged Girls

In the current issue of the New England Journal of Medicine (NEJM) is a study that tracked the weekly physical activity levels of 1,213 African American girls and 1,166 Caucasian girls over a period of 10 years.  The girls all entered the study at age 9 or 10, and were 18 or 19 years of age at the conclusions of the study.  Using a standardized measure of physical activity referred to as metabolic equivalents (MET), the study’s authors found that the black girls exerted a median of 27.3 METs per week at the beginning of the study, while he white girls expended a median of 30.8 METs per week.  Ten years later, the black girls reported a leisure-time median of 0 METs per week (a 100% decline), while the white girls reported 11 METs per week (a 64% decline).  By age 16 or 17, 56% of the black girls and 31% of the white girls reported no regular periods of exercise activity outside of school.  Lower levels of parental education were associated with a greater decline in physical activity among the white girls at both the younger and older age ranges during the course of the study.  For the black girls, parental education levels were associated with lower physical activity levels only among the older girls.  Teen pregnancy was significantly associated with decreased physical activity levels among the black girls, but not among the white girls.  On the other hand, cigarette smoking among white girls correlated with reduced levels of activity, but not for black girls.  Obesity was strongly correlated with decreased levels of physical activity among both black and white girls. 

This study identifies a dramatic decline in leisure-time physical activity levels as girls pass through adolescence, and is particularly striking among the African American girls who participated in this National Heart, Lung and Blood Institute study.  While not all of the potential contributing factors to a decline in activity were addressed by this one study, it does, however, suggest that certain potentially modifiable behaviors appear to be involved.  Pregnancy, smoking, and progressive weight gain are the three areas that might be targeted in an effort to reduce physical inactivity.  I suspect, however, that these three behaviors may, themselves, often result from other root causes.  The idealization of the female body in our culture has resulted in an epidemic of eating disorders and suicide attempts (and other self-esteem related symptoms) among teenage girls who find themselves unable to live up to society’s expectations.  Households with two working parents, or single parent households, require teenagers to act with greater independence, and often with  lesser supervision than in previous eras.  Teenage pregnancy and smoking no longer carry the social stigma that they did 10 or 20 years ago.  The gradual marginalization of physical education classes in public schools, and particularly for middle school and high school girls, has further reduced both the motivation and the opportunity for young girls to develop the habit of engaging in regular exercise.  Finally, the same factors that have resulted in over 60% of adult Americans becoming overweight are also affecting our youngsters.  Harried lifestyles combined with easy access to cheap high-fat high-calorie food, the prevalence of effort-saving appliances, and a growing preference for leisure-time activities that are sedentary in nature are all contributing factors to inactivity and obesity.

Walking, Women & Cardiovascular Disease

There is a large body of evidence to support the cardiovascular benefits of regular and moderate exercise in men.  Women, who tend to experience fewer heart attacks than men during middle age, have not been studied as extensively as men.  In this week’s NEJM is a study that compared the cardiovascular effects of walking with more vigorous exercise in 73,743 postmenopausal women aged 50 to 79 years.  The study also addressed the impact of prolonged sitting upon cardiovascular health.  All study volunteers were free of known cardiovascular disease or cancer at the outset of the study. 

Not surprisingly, increasing levels of physical activity were associated with a decreased incidence of heart attack and other signs of coronary artery disease, and these results held up for both black and white women irrespective of age or body mass.  The women who were in the top 20% in terms of weekly physical activity experienced a 53% reduction in the clinical incidence of coronary artery disease when compared to the women in the bottom 20%.  Frequent walking and vigorous aerobic exercise were both associated with a similar degree of reduction in the risk of cardiovascular disease.  A brisker walking pace and a fewer number of hours spent sitting were also predictive of a lower risk of cardiovascular disease.  Bottom line: even relatively modest regular exercise, such as walking at a brisk pace, can pay large dividends in terms of cardiovascular health.  On the other hand, too much time spent glued to a chair or sofa will significantly increase your risk of coronary artery disease.

Recently, the Women’s Health Initiative study reported that, contrary to previous belief, estrogen replacement therapy (at least when combined with progesterone) appears to actually increase the risk of coronary artery disease.  Previous studies have found no apparent reduction in the incidence of heart disease among women taking estrogen alone.  One mechanism that has been postulated whereby estrogen replacement therapy might increase the risk of heart disease is through an increase in blood levels of C-reactive protein (CRP).  CRP is a protein that plays an important role in the inflammatory process, and elevated levels of CRP in the blood are now known to be linked to an increased risk of developing coronary artery disease (in addition to directly lowering cholesterol and LDL levels, the statin drugs also appear to lower levels of CRP in the blood).  As reported in the current issue of the journal Circulation, a study was performed that compared the effects of conjugated oral estrogen pills (the most common form of estrogen replacement taken by postmenopausal women) with a skin patch that slowly releases a form of estrogen known as estradiol.  A total of 189 women were included in the study, and were randomized to receive either the oral estrogen pills or the transdermal estradiol patch.  All women also received a progesterone pill as well, to protect them from developing estrogen-induced uterine cancer.  In the women receiving the oral estrogen pills, CRP levels increased, on average, by 48% after 6 months.  The women who received the estradiol patch, however, experienced a more modest 10% increase in CRP levels.  At one year, the women receiving oral estrogen had CRP levels that were 64% higher than were present at the beginning of the study, while the women who used the estradiol patch experienced a 3% increase in blood CRP levels.  While these results do not prove that oral estrogens increase the risk of heart disease solely through elevated CRP levels (or that transdermal estradiol can prevent this adverse estrogen-related effect), this study nonetheless strongly suggests that such a mechanism may in fact be in play.  The scientific evidence linking rising blood levels of CRP with increasing incidences of heart disease is expansive and solid.  Whether or not the transdermal form of estrogen actually minimizes the risk of estrogen-related heart disease requires additional study.  The impact of the transdermal estradiol patch on the risk of breast and ovarian cancer, also needs to be thoroughly studied as well. 

Dr. Robert A. Wascher